Chapter 23 - Capacitance

What can we do to a capacitor to make the capacitance the largest possible?

1) Make the area A as large as possible. 2) Make the plate spacing d as small as possible. 3) Filling the space between the plates with a dielectric which has a relative permittivity as large as possible.

What does the magnitude of charge depend on in a capacitor?

1) The plate surface area. 2) The separation between the plates. 3) The area of overlap.

Why does a dielectric result in more charge stored on the plates?

1) The positive side of the dielectric attracts more electrons from the battery to the negative plate. 2) The negative side of the dielectric pushes electrons back to the battery from the positive plate.

How does a dielectric work?

1) When a charge is applied to the capacitor, an electric field is generated. 2)The molecules in the dielectric are polarised. 3) The surface of the dielectric near the positive plate gains negative charge, and the other surface gains positive charge. 4)The molecules each have their own electric field, which in this alignment now opposes the electric field of the capacitor. 5) The larger the permittivity, the larger the opposing field is. 6) This reduced the overall electric field, which reduces the potential difference needed to charge the capacitor - so the capacitance increases.

How do we work out the effective capacitance of two capacitors in series?

1/Ceff = 1/C1 + 1/C2.

What is the capacitance for a parallel-plate capacitor with dielectric filling the space between the plates?

C = Ae0Er/d

Write an equation for capacitance in terms of Q and V.

C = Q/V

Write an equation for C in relation to d (the distance between the two capacitor plates).

C is directly proportional to 1/d.

Write an equation for C in relation to the Area of Overlap (A).

C is directly proportional to A.

Write and equation for C in relation to the Dielectric.

C is directly proportional to the dielectric.

How do we work out the effective capacitance of two capacitors in parallel?

Ceff = C1 + C2.

What are the two equations for energy?

E=1/2CV^2 E=1/2QV

What happens when a dielectric is placed between two oppositely charged parallel plates connected to a battery?

Each molecule of the dielectric becomes polarised.

What is a Dielectric?

Electrically insulating materials that increase the ability of a parallel-plate capacitor to store charge when placed between the plates.

How do we work out the relative permittivity of a material?

Er = E1 / E0 , where Er is the relative permittivity of material 1, E1 is the permittivity of material 1 in Fm-1, E0 is the permittivity of free space.

What is the equation for relative permittivity?

Er = Q/Q0 Er = C/C0

What do we measure capacitance in?

Farads (F)

What happens to the capacitance when we decrease the distance between the two plates?

It increases.

Which terminal do the electrons move from in a capacitor battery?

Negative.

What must we be careful to do when moving the plates closer together?

Not move them too close together, as there is a minimum separation. Otherwise, the insulator is broken down from the pushback of the electrons and the strength of the electric field.

How do we find the energy stored in the capacitor?

The area under a QV graph.

What is capacitance?

The charge stored per unit pd of a capacitor.

What is another name for relative permittivity?

The dielectric constant.

What is relative permittivity?

The ratio of charge stored with the dielectric to the charge sorted without the dielectric?

What is 'RC'?

The time constant.

What is the charge on a capacitor directly proportional to?

The voltage.